How I'm thinking about our house, transportation, food and waste, to minimize environmental impact, while improving quality of life and having fun.

October 2011

10/14/2011

I've been a bit lax in posting but not because I haven't been working on the house! I've just about finished a new woodshed designed to hold two cords of wood, with the idea that we use one cord annually and the other is getting well-dried.

Once we said bye-bye to the Buderus boiler, we needed a solution to making our domestic hot water (DHW). I tossed in a 10 gallon electric water heater I had as a temporary solution. Jill and I needed to time our showers with other uses of DHW or we'd get chilly. We went along for a few weeks like this.

Once we eliminated fossil fuels as the fuel source for making DHW, our choices narrowed to using electricity or solar. In some cases, such as our former home in NH, biomass can be an option. We used a heat exchanger in our old pre-EPA VT Castings woodstove and a thermosyphon solar water heater with a back-up electric resistance element in that system. More info here. But that solution today either requires using a dirty old stove, or possibly finding and importing a European woodstove that includes a DHW coil while retaining a clean burn. As I wrote in The Right Target part 2 I didn't anticipate that we would be burning wood the whole heating season, but rather use wood where it makes the most sense - during the coldest portion of the year - and use the heat pump in the swing seasons, where it is more efficient and keeping a stove going is challenging. The restricted time of use makes wood-fired DHW less compelling.

I decided that whatever technology we would use to heat water would require a sizable, very well insulated tank, so I ordered an 85 gallon Marathon electric water heater. The Marathon has a polybutylene inner tank liner, which is durable enough for the manufacturer to offer a lifetime warranty. The insulation is continuous closed cell foam, and the outer jacket is plastic, so no rust. The Marathon heater doesn't come with an internal heat exchanger, unlike tanks designed to be heated with an external heat source such as a boiler, solar thermal collectors, or stand-alone heat pump, so I knew that whatever I did I would have to implement a heat exchanger as part of the solution (if I chose to do something other than use the Marathon as an electric resistance water heater.)

One aspect of the decision about what technology to choose to make DHW is how much hot water is needed. I lived in two successive houses I had built, for thirty years, with passive solar hot water systems that were built into the design of the homes and used no pumps, power, controls, or antifreeze. These systems can't be easily retrofitted into a conventional home. This means that a solar thermal system will typically have some of the above-mentioned cost-and-complexity adding components, which raises the cost. Professionally installed solar thermal DHW systems in New England seem to end up costing $6,000 to over $12,000, depending on size and difficulty of installation. Some of that cost is relatively fixed, regardless of system size, and therefore the economics of solar DHW is dependent on the DHW usage (and of course, the cost of energy - natural gas is much less costly per energy delivered than the fuels available on MV, which are fuel oil, propane, and electricity). Jill and I use an average of 13.3 gallons of DHW at 120F per day. With an average annual incoming water temperature of 60F (an estimate, may be somewhat lower over the year, I just have summer and early fall data) this is just about 2 kWh/day of DHW load. Add to that perhaps 1-1/2 kWh/day of heat loss off of the tank and piping near the tank (it's insulated - we'll get to this), for a total of 3-1/2 kWh/day. This is close - we were using about 3 kWh/day into the much smaller and much worse insulated 10 gallon electric water heater we used for a few weeks. So 3-1/2 kWh/day is just under 1,300 kWh/year, at a cost on MV of just under $250. This 1,300 kWh/year is the annual output of roughly 26 sf of good collector, and if we did have this small collector it wouldn't make all the DHW we need because it would make more than needed in the summer and less in the winter. But a single 26 sf collector is a very small system that still needs mounting racks, piping from the collector from the roof to the basement, pumps, controller, heat exchanger, pipe insulation, and installation labor - plumbing, wiring, solar installing - which don't scale with system size. If we were a larger family and, instead of using a somewhat European amount of 6.6 gallons/person/day, we used 15-20 gallons/person/day, we'd be figuring on an 80 sf system that might cost less than twice as much as the 26 sf system, and would deliver more than three times the usable energy - very different economics.

I looked at some interesting and innovative solar thermal systems in this process, and I'll write about them in the next post. I didn't initially choose solar DHW, in any case, so there's a bunch more to say.